1. Field of the Invention
The present invention relates to a navigation system to be mounted in a motor vehicle.
2. Description of Background Information
Recently, on-board navigation systems (navigation system to be mounted in a vehicle) for motor vehicles are developed, in which map data in digital form is stored in a memory and a map of a region around the present location of the vehicle is displayed on a display by reading-out, from the memory map, data of the region having a given area including the present location of the vehicle while recognizing the present location of the vehicle, and the location of the vehicle itself is automatically indicated on the map being displayed.
Since the map data is treated in numerical form in these navigation systems, a semiconductor memory having a large memory capacity and a display unit operable at high speeds are required in order to display a map on a display screen on the basis of the map data. Moreover, the map data in numerical form has a considerable data amount in order to satisfy various demands. Therefore, after the rise-up of the power supply of the vehicle with the so-called accessory switch of the vehicle on, the access to the memory is required quite a number of times in order to read-out the map data around the vehicle. Since a fairly long time is consumed by this operation, a time period is required from the rise-up of the power supply before the map is displayed on the screen.
Moreover, since the map data is inherently a two-dimensional data in these navigation systems, it is necessary to divide a map into parts by grids and to store the map data of each part in succession following an order indicated by the arrows in FIG. 1 so that the map data is arranged in one-dimensional manner in the memory. If the stored data is expanded with respect to the time axis, the map data is designated as shown in FIG. 2A. In this figure, numerals .circle.1, .circle.2, .circle.3, and .circle.4, represent a data group on a particular reduced scale, and numerals .circle.1', .circle.2', .circle.3', and .circle.4' represent a data group on another reduced scale. Thus, a plurality of map data on a plurality of reduced scales constitute one data group for each part defined by grids, and the map data is stored in the memory in blocks of each data group. Therefore, as is apparent from the FIG. 2A, the map data of each part defined by grids are stored in the memory at locations separated with each other in terms of the time axis.
Therefore, if the display of a map is to be made around a place, it is necessary to fetch related map data from dispersed memory locations. Moreover, since the map data carries a large amount of information, a considerably long time is required before completing to fetch all of the related data and displaying the map subsequently. Thus, the display of the map is not started until a period of time elapses after the rise-up of the power supply of the vehicle with the accessory switch on.